I can't see Portis sitting this game out for a bunch of reasons. First off, he hardly practices anymore because Zorn doesn't want to wear him out. He didn't practice much at all last year, even early on when he was a stud. He was questionable for almost every game. I wouldn't start worrying until he leaves games early.
Secondly, Zorn knows the team needs confidence bad. I see they doing everything they can to drive Detroit into the ground this week. As stated earlier, Portis has to have a great game on Sunday or else everyone is pretty much going to give up on him and the Skins. I have confidence he's going to start, and I HOPE he has a good game.